Steve Jobs, Apple's chief executive, who has survived a rare pancreatic cancer, is taking a medical leave of absence from the company almost exactly two years after he took a similar one, during which he received a liver transplant.

The news was announced when US stock markets were closed for a public holiday, almost certainly saving the company's stock from a precipitous fall. In after-hours trading in Frankfurt on Monday it fell $22, or 9%. When Jobs took a similar leave of absence in 2009, the stock was briefly suspended after falling 10% on the US market.

Apple's remaining executives are sure to face a grilling on Wednesday night when announce the company's quarterly results – though those are expected to report booming numbers from its Christmas quarter, with its iPhone and iPad products, as well as its Macintosh computers, all forecast to have sold well.

The the company's chief operating officer, Tim Cook, who has been at the company since March 1998, will take over the day-to-day operations of the company – though in an email to staff Jobs said that he will "continue as CEO and be involved in major strategic decisions for the company". He added: "I love Apple so much and hope to be back as soon as I can. In the meantime, my family and I would deeply appreciate respect for our privacy."

Apple executives in Europe declined to provide any further detail on the reason for Jobs's absence or its expected length.

Many investors and analysts see Apple as Jobs's creation – and him as the engine of its progress. He is cited as the person through whom final design decisions flow, and in whom the "DNA" of the company resides. Every new product that comes out of Apple has Jobs's fingerprints on it – usually literally. In March 2010, Barron's attempted to estimate Jobs's value to Apple, and came up with a figure of $25bn – though that will be now be substantially higher.

Jobs was diagnosed with islet cell neuroendocrine cancer of the pancreas in 2004 – a rare, and treatable form of the disease. After some delay, he was treated in summer 2004 and seemed to make a full recovery. But in January 2009 he took medical leave, and in April 2009 received a liver transplant.

But Professor John Neoptolemos, a pancreatic cancer specialist from Liverpool University, said on Monday night that for most patients in those circumstances a liver transplant is a mistake, because it requires the use of immunosuppressant drugs to prevent tissue rejection – and that can in turn allow the cancer to return in the transplanted liver and other parts of the body. There are treatments available, including radiotherapy and new drugs produced by Pfizer, which he said "should help stabilise" a patient in that situation.

Professor Neoptolemos emphasised that he has not seen any of Jobs's medical details, and that the medical literature is unclear about survival rates following liver transplants in the wake of islet cell cancer.

During Jobs's absence, Apple will be run by Tim Cook, his chief operating officer, who ran the company in 2009 while Jobs was away for six months due to the transplant.

Carolina Milanesi, vice-president of mobile devices for the research company Gartner, said that Jobs's absence need not mean the end of Apple. "The timing of the announcement [when markets were closed] shows that timing is part of being good at your job. I think that this time around, compared to 2009, people know that Tim Cook can do a good job. As a consumer, I don't care who's running the company – I just care about the products I can buy."

She added: "It's true that Jobs is the captain of the ship. And if you lose the captain for a period, then you are lost for a while. But then there are other people who can steer the ship to a safe harbour."

Apple has firmly resisted attempts to set out how it will find a chief executive to succeed Jobs. In a regulatory filing earlier this month setting out plans for its shareholders' meeting on 23 February, the directors recommend voting "against the shareholder proposal entitled 'Amend the company's corporate governance guidelines to adopt and disclose a written CEO succession planning policy'".

That recommendation will have a particular poignancy at the meeting next month – and may even find shareholders abruptly seeking clarification on precisely those lines.

The signoff in the email sent to all employees contrasts with the wording of his 2009 email, when Jobs signed off saying that he would "look forward to seeing you all this summer". He returned in June. This one is more open-ended.